1. Field of the Invention
This invention relates to writing instrument cartridges and, more particularly, to a writing instrument cartridge having a loose fitting ball follower partially immersed in a column of paste like ink exhibiting a yield value and compressed gas contained between the ink and a sealed end of the cartridge.
2. Description of the Prior Art
Known writing instrument cartridges adapted to store and dispense ink on a writing surface include a tubular reservoir connected to a writing ball and socket assembly at one end. A paste like ink is pumped into an open end of the reservoir to form an ink column. The writing ball is rotatably held in the socket and is used to collect and transfer ink as it is rolled across the writing surface.
For the purpose of exerting pressure on the ink, a solid follower in the form of a cylinder or disc is sometimes disposed to fit tightly in the reservoir between the back of the ink column and compressed gas contained in a sealed end of the reservoir. The tight fitting cylindrical follower effectively separates the compressed gas from the ink column whereby the compressed gas forces the solid follower against the back of the ink column to move the ink toward the writing ball and against the reservoir to clean ink from the reservoir walls during the writing process. Thus, the solid follower is intended to move freely and force the ink from the reservoir in response to pressure during the writing process. However, tight fitting solid followers tend to stick in place if the reservoir is dented, whereby the follower can no longer exert pressure on the ink head and the writing instrument ceases to function as intended. Loose fitting cylindrical followers having various end shapes were substituted for the tight fitting solid followers in a pressurized reservoir containing an ink exhibiting a yield value with unsatisfactory results. The compressed gas would sometimes bypass the loose fitting cylindrical follower leaving a heavy buildup on the reservoir wall. The ink in such a writing instrument would not be entirely dispensed during the writing process.
Accordingly, it is desirable to arrange a pressurized writing instrument cartridge to contain an ink with a yield value so as to eliminate the problem of the compressed gas bypassing tight fitting followers to leave a heavy residue on the reservoir wall.